Known dry-cleaning processes consist of wash, rinse, and drying cycles with solvent recovery. Garments are loaded into a basket in a cleaning drum and immersed in a dry-cleaning fluid or solvent, which is pumped into the cleaning drum from a storage tank. Conventional dry-cleaning fluids include perchloroethylene (PCE), petroleum-based or Stoddard solvents, CFC-113, and 1,1,1-trichloroethane, all of which are generally aided by a detergent. The dry-cleaning solvent is used to dissolve soluble contaminants, such as oils, and to entrain and wash away insoluble contaminants, such as dirt.
The use of these conventional dry-cleaning solvents poses a number of health and safety risks. At least one of these solvents, PCE, is a suspected carcinogen. Moreover, halogenated solvents are known to be environmentally unfriendly. To avoid these problems associated with the conventional solvents, dry-cleaning systems which utilize dense phase fluids, such as liquid carbon dioxide (CO.sub.2), as a cleaning fluid have been developed. A dry-cleaning apparatus and method employing liquid CO.sub.2 as the dry-cleaning fluid is disclosed in U.S. Pat. No. 5,467,492 entitled "Dry-Cleaning Garments Using Liquid Carbon Dioxide Under Agitation As Cleaning Medium." A similar dry-cleaning apparatus is disclosed in U.S. Pat. No. 5,651,276.
The CO.sub.2 liquid used in these dry-cleaning systems is typically stored in a storage tank and injected into the cleaning vessel during a cleaning operation. To maintain its liquid form, the CO.sub.2 in the storage tank has to be maintained under high pressure. Accordingly, the storage tank must be sealed and constructed with a thick, heavy-walled, structure to withstand the elevated pressure. The sealed structure of the storage tank and the high pressure therein make it difficult to directly monitor the level of the CO.sub.2 liquid in the tank.
It has been proposed to use an array of point sensors disposed in the storage tank to detect the liquid CO.sub.2 level. The point sensor, which may be of any of various known types, such as a temperature sensor or photo-conductivity sensor, provides a signal indicating whether the liquid level is up to the position of the sensor. Since such point sensors only sense discrete or specific levels of the liquid, they are not effective for monitoring continuous variations of the liquid level. Although improved liquid level monitoring can be achieved by providing a greater number of point sensors in the tank, such approach has the inherent disadvantage of increasing the cost of the dry-cleaning apparatus.